Energy storage systems are crucial for extensive deployment of renewable energy, such as solar and wind, considering the intermittent nature of these energy sources. Redox flow batteries (RFB) are regarded as one of the most promising technologies not only due to their ability to store large amounts of power and energy, but also to improve the efficiency of grid transmission and distribution. Unlike conventional secondary batteries, the energy of RFBs is stored in two separate tanks, each having an electroactive species, salt(s), and a solvent. The conversion between electrical energy and electrochemical energy occurs as the liquid electrolyte solutions are pumped from the storage tank to the electrodes in a cell stack. Since the energy of redox flow batteries is supplied from externally stored electrolytes, it offers flexibility in terms of energy, power, quick response and safety concerns.
Limited by the water electrolysis potential window (typically 1.2-1.6 V) and the concentration of the active material, traditional aqueous RFBs are hindered by low energy density profiles. As an emerging energy storage system, non-aqueous RFBs have attracted attention due to a wider operational potential window which directly impacts the system energy and power density.
In contrast to their aqueous counterparts, only a few non-aqueous flow batteries have been reported. The majority of the reported non-aqueous flow batteries are anion-exchange systems which employ single electrolytes composed of metal-centered coordination complexes.